In some Institute of Electrical and Electronics Engineers (IEEE) 802.11 infrastructure modes of operation, an access point (AP) transmits a beacon on a fixed channel called the primary channel. This channel is the operating channel of the BSS. This channel is also used by the stations (e.g. Wireless/Wired Transmit/Receive Units (WTRUs)) to establish a connection with the AP. The fundamental channel access mechanism in an 802.11 system is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). In this mode of operation, every WTRU, as well as the AP, will sense the primary channel. If the channel is detected to be busy, the WTRU backs off. Hence only one WTRU may transmit at any given time in such a configuration. In other 802.11 infrastructure modes of operation there may be downlink multi-user multiple-input and multiple-output (MU-MIMO) transmission to multiple WTRUs in the same symbol time frame. The WTRUs involved in MU-MIMO transmission with the AP must use the same channel or band, which may limit the operating bandwidth to the smallest channel bandwidth that is supported by the WTRUs which are included in the MU-MIMO transmission with the AP. Additionally, WTRUs in a MU-MIMO scheme may belong to different transmit power classes, may have different processing capabilities, and may have timing offsets, which may result in poor uplink MU transmission. Classification configuration, silencing protocols, timing synchronization, and trigger padding for Uplink Multi-User transmissions may address shortcomings of existing 802.11 modes of operation.